Bluetooth: Fix userspace compatibility issue with mgmt interface

To ensure that old user space versions do not accidentally pick up and
try to use the management channel, use a different channel number.

Reported-by: Keith Packard <keithp@keithp.com>
Acked-by: Johan Hedberg <johan.hedberg@intel.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo Padovan <gustavo@padovan.org>